The present invention relates to a process for the production of oxonitrides with a perovskite structure of the formula LnTaON.sub.2 (where Ln is a rare-earth element) by means of annealing over a number of hours a powdery mixture containing a tantalum (V) compound and a rare-earth metal compound in a reducing atmosphere that contains ammonia. As compared with the previously known processes of this type, the process according to the present invention unexpectedly provides oxonitrides of enhanced brightness. In another aspect, the invention also relates to the brighter oxonitrides of the formula LnTaON.sub.2 that can be obtained with the process of the present invention. Finally, the invention also relates to the use of the oxonitrides obtained by the present process as yellow-orange to reddish brown pigments of enhanced brightness.
Depending on their application and the use of the particles they color, coloring substances and pigments are subject to the most varying impediments. For example, acid or alkaline solutions may release toxicologically questionable components consisting of spinels containing nickel, cobalt, zinc, or chromium, or of cadmium sulfide yellow or cadmium sulfoselenide red or orange. A further problem is the release of toxicologically questionable heavy metals from these kinds of pigments during the incineration of the plastics colored by them in garbage incineration plants. There exists, therefore, a special interest in enlarging the palette of inorganic yellow-orange and reddish brown pigments whose components are more acceptable from the toxicological viewpoint.
German patent application P 42 34 938.9 (U.S. patent application Ser. No. 08/135,998, filed on Oct. 14, 1993, now U.S. Pat. No. 5,376,359, which is incorporated by reference in its entirety) teaches a process for the production of tantalum (V) nitride by nitration of an oxidic tantalum (V) compound with dry ammonia at 750.degree. to 950.degree. C. and the use of the resulting tantalum (V) nitride as a pigment for coloring plastics and lacquers. Tantalum is viewed as toxicologically harmless.
Known from FR-A 2 573 060 (U.S. Pat. No. 4,734,390 which is incorporated by reference in its entirety) are nitrides and oxonitrides of the general formula ABO.sub.3-n N.sub.n which exhibit a perovskite structure. The general formula includes oxonitrides in which A is a rare-earth element, B is tantalum, and n is the number 2. Compounds of this type are produced by annealing a powder mixture of an oxide, oxonitride, or nitride of element A and an oxide, oxonitride, or nitride of element B in a nitrogen or ammonia atmosphere. Proceeding from the oxides of element A and element B, the specified annealing period is 48 hours, the annealing temperature is about 1000.degree. C., and the reducing gas for nitration is ammonia. The document specifies an end use involving only dielectrical materials and makes no reference to the color of the individual oxonitrides or their use as pigments.
In repeating the known process referred to above, it was established that the resulting oxonitrides of the general formula LaTaON.sub.2 basically exhibit brown shades of color which, however, have little brightness and are chromatically unattractive. Also working to a disadvantageous effect in the known process are the long reaction time, the high temperature, and the high flow speed of the ammonia acting as the nitration agent.